A liquid crystal display device has been widely used as a thin, light-weight, and low power consumption display device. A liquid crystal panel included in the liquid crystal display device has a structure formed by attaching an active matrix substrate and a counter substrate together, and providing a liquid crystal layer between the two substrates. A plurality of gate lines, a plurality of data lines, and a plurality of pixel circuits each including a thin film transistor (hereinafter referred to as TFT) and a pixel electrode are formed on the active matrix substrate.
As a system for applying an electric field to the liquid crystal layer of the liquid crystal panel, a vertical electric field system and a lateral electric field system are known. In a liquid crystal panel of the vertical electric field system, an almost vertical electric field is applied to the liquid crystal layer by using the pixel electrode and a common electrode formed on the counter substrate. In a liquid crystal panel of the lateral electric field system, the common electrode is formed on the active matrix substrate together with the pixel electrode, and an almost lateral electric field is applied to the liquid crystal layer by using the pixel electrode and the common electrode. The liquid crystal panel of the lateral electric field system is advantageous in having a wider view angle than the liquid crystal panel of the vertical electric field system.
As the lateral electric field system, an IPS (in-plane switching) mode and an FFS (fringe field switching) mode are known. In a liquid crystal panel of the IPS mode, the pixel electrode and the common electrode are each formed in the shape of comb teeth, and are disposed so as not to overlap each other in a plan view. In a liquid crystal panel of the FFS mode, a slit is formed either in the common electrode or the pixel electrode, and the pixel electrode and the common electrode are disposed so as to overlap each other via a protective insulating film in a plan view. The liquid crystal panel of the FFS mode is advantageous in having a higher aperture ratio than the liquid crystal panel of the IPS mode.
An active matrix substrate in the liquid crystal panel of the FFS mode is manufactured using five or six photomasks. Patent Document 1 describes a method for manufacturing the active matrix substrate using five photomasks. In the manufacturing method described in Patent Document 1, a photomask for a semiconductor layer is not used, a semiconductor layer is patterned using a photomask for a source layer, and a channel region of a TFT is formed using a photomask for a pixel electrode layer. In the active matrix substrate described in Patent Document 1, a data line is a laminated wiring formed in the semiconductor layer, an ohmic contact layer, the source layer, and the pixel electrode layer. Portions of the data line which are formed in the semiconductor layer, the ohmic contact layer, and the source layer have the same shape, and a portion of the data line which is formed in the pixel electrode layer is formed so as to cover the portions formed in the three layers.